1. Field of the Invention
The present invention generally relates to the field of optical fibers, in particular the present invention is directed to a new and novel method and apparatus for curing optical fibers with ultraviolet radiation.
2. Discussion of Related Art
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with relatively low signal loss as compared to standard wire or cable networks. The use of optical fibers in today""s technology has developed into many widespread areas, such as: medicine, aviation, communications, etc. Because of this development, there is a growing need to produce optical fibers of better quality at faster rates and lower costs.
Many of the areas of use for optical fibers, such as communications, require the optical fibers be protected from various destructive elements, such as adverse weather, moisture, impact damage, etc. This protection for the individual fibers comes from the fiber coatings. Today, most optical fibers have two coatings, which are often referred to as the primary and secondary coatings. The primary coating is applied onto the surface of the optical fiber, with the secondary coating being applied on top of the primary coating. The main function of the primary coating is to provide a soft xe2x80x9ccushionxe2x80x9d for the glass fiber, protecting it from shock damage. The main purpose of the secondary coating is to provide a semi-rigid protective shell to protect both the primary coating and the glass fiber from adverse environmental elements, as well as physical damage.
Often these coatings have ultraviolet (UV) photoinitiators in their composition. Photoinitiators function by absorbing energy which is radiated by a UV, or sometimes a visible, light source. This energy absorption then initiates polymerization of the liquid coating placed on the fiber, and accelerates the hardening of the coating. This acceleration greatly reduces the production time of optical fibers, making production more profitable. The curing of the coatings take place in specific UV curing stages during the optical fiber manufacturing process. There are curing xe2x80x9cchambersxe2x80x9d at these stages which facilitate the curing of the coatings.
Within most of these curing xe2x80x9cchambersxe2x80x9d there is at least one UV light source which emits UV radiation or light onto the optical fiber coating. It should also be noted that this curing step is also found when manufacturing fiber optic cables or ribbons, when the cable or ribbon matrix or substrate is to be cured around a plurality of fibers. In either application, the basic function and operation of the UV curing chamber remains the same.
As stated earlier, most prior art curing chambers have at least one UV light source or bulb to emit the UV radiation or light. This configuration leads to a very serious problem inherent in prior art curing chambers. Optical fibers, fiber ribbons and fiber cables require curing around the complete circumference of their coatings or matrix materials. Because the coatings are applied concentrically around the fibers, ribbons or cables, the entire 360xc2x0 around the center line of the fiber, ribbon or cable must be cured evenly. Without an even cure of the coating there will be uneven cure gradients throughout the coating, which leads to inadequate protection of the fibers.
In an effort to avoid the problem of uneven cure around a fiber, ribbon or cable, prior art curing devices use mirrors or reflective surfaces inside the curing chamber to reflect the UV radiation from the bulb back at the surface or coating to be cured. Although this partially addresses the problems associated with the prior art devices, it is a relatively inadequate solution. Primarily, the problems associated with uneven cure gradients in the coatings are not completely avoided. The mirrors or reflectors that are used are not 100% efficient. This means that some of the UV radiation or light emitted from the bulb, which strikes the reflective surface of the mirror, is either absorbed into the surface, or refracted or reflected away from the coating or substrate to be cured. Therefore, some of the UV radiation from the light source is lost and/or not directed at the coating to be cured. Because of this the intensity of radiation is different around the coating or substrate. This difference in intensity translates to different cure states around the coating or substrate, and as stated earlier, this is highly undesirable.
Another significant problem associated with current UV curing chambers is the relatively high heat that is generated from the UV lamps during operation. In most prior art curing chambers the UV light source used does not exclusively emit UV light, but also emits other wavelengths of light like those found in the infrared light spectrum. The infrared (IR) light emitted generates a significant amount of heat in the curing chamber during operation. The generation of this heat leads to a number of problems in the manufacture of optical fibers, ribbons and coatings.
A major problem is due to the fact that the heat generated is added to the heat which already exits from the optical fiber being drawn through a furnace (used to draw a preform into a fiber). This added heat accelerates the curing of the coatings on the fibers, ribbons, and cables, and can lead to the coatings being xe2x80x9cover-curedxe2x80x9d or improperly cured. xe2x80x9cOver-curexe2x80x9d is a situation where the coating or substrate becomes too hard and leads to microbending in the optical fiber which adversely affects the quality of the signal sent through the fiber.
One of the most common solutions used in the prior art to address the problems associated with the high heat generated, is through the use of cooling air in the UV curing chamber. However using cool air to try and control the high heat levels, that can be reached, is not without its problems. First, if air is passed over the coating or substrate being cured the oxygen in the air inhibits proper polymerization and, therefore, proper curing of the coating or substrate. Second, even if the air is not in contact with the coating, but is passed through a cooling xe2x80x9ctubexe2x80x9d the air inhibits the transmission of the UV radiation to the substrate. Air tends to absorb, reflect, and/or refract a percentage of the UV radiation emitted from the light source used in the chamber. This coupled with the loss of UV radiation in the reflective surfaces (mirrors) used greatly reduces the efficiency of the UV light source requiring more power in the UV light source (and thus more heat) or slowing the curing process. Third, and perhaps most importantly, when UV radiation is passed through air harmful ozone is created through a chemical reaction between the UV radiation and oxygen. The creation of ozone is extremely disadvantageous due to the costs and complexity of the measures required to protect the environment from this ozone.
There have been efforts in the prior art to address some of the problems discussed above, but they fall short. For example, the German Patent DE 39 13519 C2 discloses a UV bulb which is tubular in shape. This allows the bulb to surround the full 360xc2x0 of the coating or substrate to be cured. This is to address the problems associated with using a single bulb with a number of reflective surfaces, which can lead to uneven curing around the coating or substrate. This patent also discloses using a UV transparent cooling medium to cool the bulb in an effort to avoid the problems discussed above. However, the patent discloses using a reflector on the exterior of the bulb to reflect radiation that is emitted out from the bulb back at the bulb and, therefore, the substrate or coating to be cured. This aspect of the patent disclosure has some serious drawbacks.
UV bulbs have a high temperature plasma region within the bulb that generates the UV radiation (among other types of radiation, such as IR). Because of this plasma region inside the UV bulb, reflected UV radiation will not be able to pass through the bulb to the substrate or coating. In fact, UV as well as IR radiation reflected back at the bulb is xe2x80x9ccapturedxe2x80x9d in the bulb, thus increasing the temperature of the bulb unnecessarily. This increase in bulb temperature adversely affects the efficiency of the cooling medium and systems used. This is because, as the operation of the bulb goes on, the bulb will be steadily increasing its own temperature as the UV and IR radiation is reflected back into the bulb and trapped there because of the plasma in the bulb. This increasing heat, and reduced efficiency of the cooling system will lead to a decrease in the operational life of the bulb and cooling system without any increase in the efficiency in the curing of the coating or substrate, as the reflected UV radiation never reaches the substrate or coating, and is trapped between the bulb and the exterior reflective surface.
The present invention is directed to eliminating the above problems associated with prior art curing chambers through the use of a new and novel ultraviolet bulb, curing apparatus and curing method.
The present invention avoids the above problems with the prior art by employing a number of new and novel features, which will be discussed briefly here, and in more detail below. First, the present invention employs a UV transparent tube, through which the coated optical fiber or substrate is passed. An inert gas is passed through the center of this tube so as to provide an inert environment free of oxygen, due to the adverse affect oxygen has on the polymerization process of UV curable coatings and substrates. This UV transparent tube and flowing inert gas further acts as an exhaust vessel to blow out volatile gases and emissions generated during the curing of the coating or substrate. This is to minimize the deposit of these volatiles on the surface of the UV transparent tube. The deposition of these volatiles tends to xe2x80x9ccloudxe2x80x9d the tube, thus reducing the efficiency of the curing process as the UV radiation is blocked and/or dissipated by the xe2x80x9cclouding.xe2x80x9d The inert gas used in this process is either heated or cooled to allow the temperature surrounding the coating to be controlled and changed to ensure optimum efficiency of the curing process.
Second, the present invention further employs a hollow tubular UV bulb which is positioned concentrically around the UV transparent tube and coated fiber or substrate to be cured. The UV tube bulb will avoid the problems in the prior art of uneven curing by providing an evenly distributed UV radiation source completely around the substrate or coating to be cured. In the present invention, the UV tube bulb is large enough to provide a gap between the inner surface of the bulb and the exterior surface of the UV transparent tube. A cooling medium is passed through the gap created between the tube and the bulb to allow active cooling of the UV bulb. However, to avoid many of the problems with the prior art the cooling medium will be of a type transparent to UV radiation, thus allowing the radiation to pass through the cooling medium, remaining unaffected by the cooling medium. This will greatly increase the efficiency over prior art curing methods.
Further, the present invention employs the use of an infrared filter coating placed on either the interior side of the UV tubular bulb or on the UV transparent tube, or both. This coating prevents IR radiation from reaching the substrate or coating to be cured, preventing many of the problems associated with overheating the coating or substrate to be cured. However, this aspect of the invention may not be necessary in situations where the restriction of heat emitted to the coating or substrate is not necessary. This shielding will further increase the efficiency of the bulb by reflecting the IR radiation back into the plasma region of the bulb, thus increasing the activation efficiency of the plasma and, therefore, the bulb""s efficiency.
Instead of using a reflector on the outside of the bulb to reflect all of the emitted radiation back into the bulb, including IR and UV radiation, the present invention employs a dichroic reflector that is transparent to most radiation wavelengths, including IR, but reflects the UV radiation back into the bulb. This will prevent the dramatic increases in bulb temperature found in the prior art, by allowing the IR radiation to escape the bulb while avoiding the environmental problems associated with the creation of harmful ozone, due to the reaction of UV radiation with the oxygen in the air surrounding the exterior of the bulb.
Finally, the present invention further increases the temperature control over the UV bulb, and thus its efficiency, by providing an exterior cooling system for the bulb. A fluid or gas cooling medium is flowed or drawn past the exterior of the bulb to provide an additional cooling effect. In an embodiment of this invention this cooling medium is the primary cooling means. By placing the majority of the cooling means on the exterior of the bulb, the flow direction of the heat transfer from the bulb will be primarily outward, not inward towards the inner cooling medium. Therefore, this exterior cooling medium aids in drawing additional heat from the UV bulb away from the coating or substrate to be cured, thus allowing more control over the curing stage and greatly reducing the risks associated with over-cure, or heat damage to the coating or substrate.